Field of the Invention
The present invention concerns a method to create an artifact-free magnetic resonance image data set, as well as a magnetic resonance system and an electronically readable data medium for executing such a method.
Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified form, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (also called a B0 field) with a field strength from 0.2 Tesla to 7 Tesla or more in a magnetic resonance apparatus, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, and the triggered nuclear magnetic resonance signals are entered into an electronic memory organized as k-space data, on the basis of which MR images are reconstructed or spectroscopy data are determined. For spatial coding of the measurement data, rapidly switched (activated) magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with values, for example by means of a multidimensional Fourier transformation.
MR examinations are most often very loud. The main reason for this that is the rapidly changing gradient magnetic fields (also called just gradients) used for the measurement, cause eddy currents, distortions and oscillations in the gradient system that is used, in particular the gradient coil that is used. This energy also transfers to the housing of the magnetic resonance system, which then likewise oscillates and therefore itself emits noise.
In order to design an MR examination to be as quiet as possible, pulse sequences for the acquisition of magnetic resonance measurement data can be used in which the changes of the gradients over time (dG/dt)—also called the slew rate—are as small as possible.
An example of such a “quiet sequence” is known as the PETRA sequence as described in the article by Grodzki et al.: “Ultra short Echo Time Imaging using Pointwise Encoding Time reduction with Radial Acquisition (PETRA)”, Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), Page 2815.
In the reconstruction of image data sets from the measurement data acquired by means of a pulse sequence, it is important to know the gradients switched in the acquisition of the measurement data—and therefore the trajectories along which k-space corresponding to the examination subject has been scanned—as optimally as possible.
Techniques known as field mapping and associated field mapping devices are known in order to measure gradient fields in the measurement volume of a magnetic resonance system. A particularly robust field mapping technique and device is described in, for example, the article by Dietrich et al.: “A stand-alone system for concurrent gradient and RF sequence monitoring”, Proc. Intl. Soc. Mag. Reson. Med. 20 (2012), Page 700.